1. Technical Field
The present invention relates to a process for preparing a propylene random copolymer. More particularly, it relates to a process for preparing a random copolymer having an unsaturated hydrocarbon moiety comprising propylene and a specified diene in the presence of a specified catalyst.
2. Prior Art
It is well known in the art that propylene homopolymers or copolymers (referred to hereinafter generally as propylene polymers) are developed for their applications to the wide range of fields owing to their excellent properties.
On the other hand, it is also known that these polymers are essentially saturated hydrocarbons and thus have problems that their graft reaction or crosslinking reaction are limited because of their inferior chemical reactivities and that they are least susceptible to an adhesive, coating, printing or the like.
In order to solve the problems, a variety of inventions have been proposed (Japanese Patent Laid-Open Publications Nos. 165907/1980, 30414/1981, 36508/1981, 115007/1987, 115008/1987 and 145611/1990). These inventions relate to the copolymers of propylene and a non-conjugated diene, which are attracting attention as reactive polypropylenes because of the presence of an unsaturated double bond in the side chains.
These proposals, however, could entail problems such that a non-conjugated diene may not always have a high copolymerization reactivity and thus have a problem that an expensive non-conjugated diene compound must be used in a large amount and the copolymer is produced in a small amount as compared with the amount of the catalyst used (i.e. low catalyst activity), so that the production cost tends to become expensive. Thus, further improvements are desired.
On the other hand, it is also well known that an .alpha.-olefin polymer is produced at a high yield in the presence of a polymerization catalyst comprising a transition metal compound having a conjugated five-membered ring ligand and an alumoxane (Japanese Patent Laid-Open Publication Nos. 19309/1984, 35007/1985, 130314/1986, 295607/1988 and 41303/1990). Also, proposals referring to a process for preparing a copolymer of propylene and a 1-olefin having 4 or more carbon atoms in the presence of these catalysts (Japanese Patent Laid-Open Publication Nos. 119212/1987, 266116/1989, 173014/1990 and 247207/1990).
The polymers obtained according to these proposals in their original forms, however, have narrow molecular weight distributions, so that their moldabilities are often deteriorated. In order to improve the moldabilities, the use of a plurality of metallocene compounds has been proposed for the broadening of the molecular weight distribution of the polymer obtained which is a mixture of polymers of different molecular weight (Japanese Patent Laid-Open Publication No. 35008/1985). However, if the polymers in the mixture produced have large difference in their molecular weights, these polymers are hardly compatible and require complicated operation controls. Therefore, it is desirable to broaden the molecular weight distribution by the polymerization in the presence of a single catalyst.
Copolymerizations of an olefin and a diene compound in the presence of a metallocene catalyst have themselves been proposed (Japanese Patent Laid-Open Publication No. 19309/1983, WO 88/04672, WO 88/04673, WO 88/04674). According to these proposals, the polymers obtained may have narrower molecular weight distributions than usual which could entail the deterioration of their moldabilities. copolymerization of propylene and a non-conjugated diene has not been proposed as far as the present inventors know.
The object of the present invention is to provide a solution to the aforementioned problems.